Bandwidth of picture signal
- 1. 140140111029 Audio and Video System Government Engineering College Bharuch
- 2. Bandwidth Of Picture Signal Prepared by Krushal Kaklotar
- 3. Channel Bandwidth In the 625 line TV system where the frequency components present in the video signal extend from dc (zero Hz) to 5MHz, a double sideband AM transmission would occupy a total bandwidth of 10 MHz. The actual band space allocated to the television channel would have to be still greater, because with practical filter characteristics it is not possible to terminate the bandwidth of a signal abruptly at the edges of the sidebands.
- 4. Therefore, an attenuation slope of 0.5 MHz is provided at each edge of the two sidebands. This adds 1 MHz to the required total band space. In addition to this, each television channel has its associated FM (frequency modulated) sound signal, the carrier frequency of which is situated just outside the upper limit of 5.5 MHz of the picture signal. This, together with a small guard band, adds another 0.25 MHz to the channel width, so that a practical figure for the channel bandwidth would be 11.25 MHz Such a bandwidth is too large, and if used, would limit the number of channels in a given high frequency spectrum allocated for TV transmission. Therefore, to ensure spectrum conservation, some saving in the bandwidth allotted to each channel is desirable.
- 5. Total channel bandwidth using double sideband picture signal. P is picture carrier and S is sound carrier
- 6. VESTIGIAL SIDEBAND TRANSMISSION In the video signal very low frequency modulating components exist along with the rest of the signal. These components give rise to sidebands very close to the carrier frequency which are difficult to remove by physically realizable filters. Thus it is not possible to go to the extreme and fully suppress one complete sideband in the case of television signals.
- 7. The low video frequencies contain the most important information of the picture and any effort to completely suppress the lower sideband would result in objectionable phase distortion at these frequencies. This distortion will be seen by the eye as ‘smear’ in the reproduced picture. Therefore, as a compromise, only a part of the lower sideband, is suppressed, and the radiated signal then consists of a full upper sideband together with the carrier, and the vestige (remaining part) of the partially suppressed lower sideband. This pattern of transmission of the modulated signal is known as VSB
- 8. COMPLETE CHANNEL BANDWIDTH The sound carrier is always positioned at the extremity of the fully radiated upper sideband and hence is 5.5 MHz away from the picture carrier. This is its logical place since it makes for minimum interference between the two signals. The FM sound signal occupies a frequency spectrum of about ± 75 KHz around the sound carrier. However, a guard band of 0.25 MHz is allowed on the sound carrier side of the television channel to allow for adequate inter-channel separation. The total channel bandwidth thus occupies 7 MHz and this represents a bandspace saving of 4.25 MHz per channel, when compared with the 11.25 MHz space, which would be required by the corresponding double sideband signal.
- 9. Complete bandwidth of( Indian ) TV channel sideband spectrum. C is colour subcarrier frequency
- 10. Thank You